In order to efficiently produce hydrocarbons from a subterranean formation, the formation must be sufficiently conductive in order to allow the hydrocarbons to flow to the wellbore. Various treatments for increasing the conductivity of a subterranean formation have been developed.
One technique for increasing the conductivity of a subterranean formation and thereby stimulating production of hydrocarbons from the formation is hydraulic fracturing. Hydraulic fracturing generally involves pumping one or more treatment fluids into the formation at a sufficient hydraulic pressure to create or enhance one or more fractures in the formation. Typically, a pad fluid that does not contain any proppant particulates is first injected into the formation to initially fracture the formation. Following injection of the pad fluid, a proppant slurry that includes a plurality of proppant particulates is injected into the formation. The proppant slurry deposits the proppant particulates in the fracture and any branches thereof in order to prevent the fracture and the fracture branches from fully closing once the hydraulic pressure from the fluid is released and the fracturing operation is complete. The resulting “propped fracture” provides a conductive channel through which fluids in the formation can flow to the wellbore. As used herein and in the appended claims, the term “propped fracture” means a fracture (naturally-occurring or otherwise) in a subterranean formation that contains a plurality of proppant particulates.
Fracturing tight formations of unconventional reservoirs, such as formations containing shale, tight sandstone formations and coal bed formations, requires special considerations. For example, shale, coal and other types of formations can have a permeability of approximately 1 millidarcy (mD) or less. Hydraulically fracturing such formations typically forms a complex fracture network that includes primary fractures (and branches thereof) and microfractures (including natural microfractures and induced secondary microfractures) in a zone of the formation surrounding the wellbore.
For example, the microfractures can extend from a tip and edges of a primary fracture or a branch thereof and extend outwardly in a branching tree-like manner from the primary fracture. The microfractures can extend transversely to the trajectory of the primary fractures allowing them to reach and link natural fracture both in and adjacent to the trajectory of the primary fractures. The microfractures can exist and be formed in both near-wellbore and far-field regions of the zone, as well as regions located adjacent to primary fracture branches. As a result, the microfractures can give more depth and breadth to the fracture network.
In the absence of proppant particulates, the microfractures tend to close back once the hydraulic pressure placed on the formation is released or decreased. Conventional or traditional proppant particulates are often too large to prop the microfractures open. As a result, due to their size, conventional proppant particulates cannot be easily placed in microfractures. Allowing the microfractures to close cuts off a significant portion of the fracture network and ultimately prevents the production of valuable hydrocarbons therefrom.
In order to address this issue, micro-proppant particulates having a size sufficient to allow the particulates to be placed in microfractures have been developed. The micro-proppant particulates are included in the pad fluid stages of the fracturing treatment. Including micro-proppant particulates in the pad fluid places the micro-proppant particulates in the fissure openings to and otherwise in the microfractures as soon as they are opened or created. By holding the microfractures open, the micro-proppant particulates help maintain fluid communication between the microfractures and the primary fractures. Conventional proppant particulates are then included in the proppant-slurry stages of the fracturing treatment and placed in the primary fractures and branches to help ensure that fluid conductive flow paths to the wellbore are maintained.
During the injection of the proppant slurry stages and even after the hydraulic pressure on the formation is released and the fractures are allowed to close on top of the proppant particulates, the base fracturing fluid continues to leak off into the complex fracture network as a whole including the opened and/or created microfractures in the network. Unfortunately, such leak off can displace the micro-proppant particulates away from the fissure openings of and deep into the microfractures. This can cut off communication of the microfractures with the overall fracture network and defeat or partially defeat the purpose of placing micro-proppant particulates in the microfractures in the first place. In tight formation where microfractures are prevalent, the inability to keep the microfractures open can significantly reduce the potential amount of hydrocarbons that can be recovered from the formation.